1. Field of the Invention
The present invention relates to a lens position detector for a camera, and more specifically, relates to a lens position detector used for a lens barrel of a camera in which a lens group moves in an optical axis direction by a rotation of a manual operating ring provided on the lens barrel, wherein the detector is configured to be capable of detecting the position of the lens group in the optical axis direction with respect to the rotation position of the manual operating ring.
2. Description of the Related Art
A photographic lens barrel of a camera which is provided with a focus ring (manual operating ring) and a zoom ring (manual operating ring) that are operated by the user to change the position of one or more movable lens groups provided in the lens barrel in an optical axis direction to thereby adjust the focus and the focal length of the photographic lens, respectively, is known in the art. Various mechanisms for changing the position of one or more movable lens groups by a rotation of the focus ring or the zoom ring have been proposed. Among such mechanisms, a mechanism wherein a cam ring provided in a lens barrel rotates via a rotation of a manual operating ring, and a lens frame which supports a lens group is provided integrally with a set of cam rollers (cam followers) which are respectively engaged in a set of cam grooves formed in a cam ring to convert rotational movements of the cam ring into linear movements of the set of cam rollers, i.e., linear movements of the lens group supported by the lens frame, is known in the art.
With this type of mechanism, a lens position detector is incorporated in a lens barrel when it is required to detect the position of a movable lens group or groups in an optical axis direction to detect a state of focusing or zooming. For instance, Japanese Utility Model Publication No. 2593855 discloses such a lens position detector incorporated in a lens barrel. The lens barrel disclosed therein is provided with a code plate having a predetermined conductive pattern thereon which is fixed to one of a stationary portion of the lens barrel and a lens support frame for supporting a lens group and moves with the lens group in an optical axis direction, and is further provided with a conductive brush which is in sliding contact with the conductive pattern of the code plate so that the conductive brush slides on the conductive pattern as the lens support frame moves relative to the stationary portion of the lens barrel. Variations in shape of the conductor traces of the conductive pattern in the direction of sliding movement of the conductive brush thereon are detected via the conductive brush that is in contact with the conductive pattern, and the variations thus detected are output as an electrical signal to detect the position of the lens group in the optical axis direction. Another type of lens position detector is known in the art which detects the position of a movable lens group by detecting an angle of rotation of either a cam ring having an intimate relationship with the position of the movable lens group in the optical axis direction or a manual operating ring integral with the cam ring using a similar combination of a code plate and a conductive brush.
Providing a code plate and a conductive brush on internal elements of a lens barrel such as a lens support frame and a cam ring in a manner similar to that disclosed in the aforementioned utility model publication complicates the internal structure of the lens barrel and limits the arrangement of elements of the lens position detector because such elements must be arranged so as not to interfere with elements of a cam mechanism such as cam rollers, which makes it difficult to design the lens position detector. Accordingly, it is conceivable to mount one or more elements of a lens position detector to a manual operating ring positioned in the radially outermost part of the lens barrel. For instance, a conductive brush can be secured to a part of the manual operating ring to rotate with the manual operating ring about an optical axis, while a code plate is fixed to a part of a stationary ring which faces the conductive brush in the optical axis direction.
This structure is effectively used in the case where the manual operating ring rotates at a specific position (fixed position) in the optical axis direction. However, in the case where the manual operating ring is screwed into a stationary ring via screw threads (or helicoidal threads) in a manner similar to those of recently-produced lens barrels, the manual operating ring moves in the optical axis direction in accordance with the lead angle of the screw threads (or helicoidal threads) when the manual rotating ring is rotated, and accordingly, this movement of the manual rotating ring in the optical axis direction changes the distance between the code plate and the conductive brush that face each other. This change varies the contact pressure of the conductive brush on the code plate, thus making it impossible to maintain an ideal state of electrical contact between the code plate and the conductive brush. Therefore, there is a possibility that a proper electrical signal, i.e., a lens position detection signal may not be obtained in the long run. Additionally, in the case where the range of rotation of the manual rotating ring is great, there is a possibility that the conductive brush may become out of contact with the code plate to thereby make it impossible to obtain a lens position detection signal.